1. Technical Field
The present invention relates generally to lithography tools, and more particularly, to electron beam lithography tool image quality evaluating and correcting.
2. Related Art
Image quality optimization on an electron beam lithography tool involves the adjustment of many electron optical parameters to correct for resolution limiting effects. Illustrative parameters include filament power, anode position, lens excitations, shape aperture rotation, shape deflection calibration, focus coil excitation, stigmator corrector excitations and beam exposure time. Contributing resolution detractors must be determined from the resultant exposed images. Resolution limiting effects may include, for example, illumination non-uniformity, shape aperture image rotation errors, shape aperture image size errors, shape deflection miscalibration, spot defocus, spot astigmatism, focus variations and dose variations.
Conventional methodology to provide image quality optimization includes implementation of a test pattern, having a specialized test pattern cell, within an exposure field to highlight potential problems, and then determination of appropriate image quality corrections of the lithography tool to address the identified problems. The magnitude of some resolution detractors, such as spot shape errors, spot illumination non-uniformities and beam dose errors are typically independent of location in the exposure field. Accordingly, correction requires evaluation of exposed images for consistent errors across the field, and then applying uniform corrections. In contrast, other resolution limiting factors such as spot defocus and astigmatism lead to image quality variations across the exposure field. Corrections for these image quality problems have to be a function of position in the exposure field. The exposure field may be made up of thousands of sub-fields (e.g., a 2.16 mm×2.16 mm exposure field can be broken up into 90×90 array of 24 micron sub-fields). Typically, exposure of the test pattern includes exposure using an identical test pattern cell at nine (9) sub-field positions of an exposure field. Assuming a square exposure field, the test pattern cell may be used at the four corner sub-field positions, vertical and horizontal intermediate sub-field positions between the four corner sub-field positions, and a center sub-field position. Focus and astigmatism corrections for sub-field positions between these nine sub-field positions are provided by linear interpolation of correction data for the two nearest sub-field positions, which introduces inaccuracy in the corrections. For example, for a sub-field position half-way between a corner sub-field position and the center sub-field position, an averaging of the corrections for those respective sub-field positions would be used.
Additional problems relative to image quality optimization result from conventional test pattern cells. In particular, each test pattern cell is typically provided with one or two unique space widths, which necessitates numerous tests to address problems with a range of space widths. For example, an illustrative test pattern cell 10 is shown in FIG. 1. Test pattern cell 10 includes a plurality of elongated spaces 11 extending from a border 14 , another plurality of elongated spaces 12 extending from border 14 , an array of varied sized square spaces 16 , and an X pattern cell 18 and a cross pattern cell 20 of the same width as elongated spaces 12. In terms of elongated spaces, only two different widths are provided. Use of such a limited number of unique elongated space widths in a test pattern cell presents problems because the choice of appropriate corrections to improve image quality may depend on the image fidelity over a range of space widths. For example, exposure dose calibration errors can result in space width errors that increase or decrease as a function of nominal feature width, or uniformly large or small features. In contrast, spot illumination problems typically cause reduced space widths for small features, but may cause no noticeable width decrease for relatively large features. Currently, the only way to compare corrections for a large range of different elongated space widths is by making multiple exposures of different test pattern cells. Separate test pattern cell exposures introduce new exposure and process variables, which complicate the correction analysis.
Another disadvantage of two width elongated space test pattern cells is that they do not allow evaluation of a lithography tool relative to beam shaping calibration. In particular, beam shaping is typically accomplished by applying a calibrated voltage to one or more electrically conductive plates adjacent to the electron beam of the lithography tool to electro-statically shift the beam over an aperture to shape (and size) the emitted beam. Where a test pattern cell provides only one or two elongated space widths, testing can only coarsely evaluate whether the lithography tool shaping deflection is accurately calibrated.
In view of the foregoing, there is a need in the art for improved methods of image quality evaluation and correction, and a test pattern cell that does not suffer from the problems of the related art.